1. Field of the Invention
This invention relates to a bidirectional photothyristor device which performs a bidirectional switching operation in response to a light-trigger signal.
2. Description of the Prior Art
A bidirectional thyristor which, for example, exhibits bidirectional switching characteristics comprises a semiconductor substrate having five continuous layers of alternately different conductivity type and including two controlled rectifier regions, in parallel opposed arrangement, which comprise four layers and have an end layer respectively formed by one or the other of the two outer layers of the five continuous layers; a pair of main electrodes in ohmic contact with the outer layers and intermediate layers adjacent thereto; and means for supplying a trigger signal. In such a device, when a trigger signal is supplied in a state that a voltage is applied between the main electrodes so that the first of the main electrodes is at higher potential than the second and one controlled rectifier region is biased forwardly, that region begins to turn on. On the other hand, when a trigger signal is supplied in a state that a voltage is applied between the main electrodes so that the second main electrode is at higher potential than the first main electrode and the other controlled rectifier region is biased forwardly, that region begins to turn on.
The most common type of means for supplying a trigger signal is what is called the electric gate system in which one of the layers of the semiconductive substrate is provided with a gate electrode from which a gate signal current is supplied to perform a switching operation of the device. The electrical gate system, however, has the following disadvantages:
1. Provision of two gate electrodes is needed for a symmetrical bidirectional switching operation. This requires two gate circuits electrically isolated from each other.
2. In case of the provision of a single gate electrode, the gate electrode is located close to both the two quadruple layer regions, and at the same time a portion of the outer layer exposed to one principal surface opposite to the other principal surface on which the gate electrode is located is formed so that its projection in the stacking direction is partially overlapped with the gate electrode. Therefore, at an instant of commutating (during which conductive state is switched from one quadruple layer region to the other quadruple layer region), a possible switching of the conduction state will be caused before the application of a gate signal current, thereby resulting in a so-called erroneous firing.
3. In order to prevent interactions between the two quadruple layer regions, an isolation region is formed between the two quadruple layer regions by using intermediate triple layer regions so that projections of both the outer layers are spaced apart from each other by a predetermined distance. The isolation region thus formed requires the provision of two gate electrodes, resulting in the same disadvantage as item (1).
On account of these disadvantages of the electric gate system, application of the conventional bidirectional thyristors was limited to fields of applications requiring neither high reliability nor a high rated value of current or voltage, in spite of capability for the bidirectional switching operation.
On the other hand, a so-called light trigger system is being highlighted today. According to this system, a trigger signal undergoes conversion into a light signal and the light signal is irradiated on the semiconductive substrate to enable the switching operation. As compared with the electrical gate system, the light trigger system has the following advantages.
1. The two quadruple layer regions electrically isolated from each other can be fired by a single light source electrically isolated from these quadruple layer regions so that the provision of a single gate circuit is satisfactory.
2. Bidirectional thyristor circuits are electrically isolated from the gate circuit so that easy application of thyristors of this type is assured to high voltage circuits. The bidirectional thyristor with light trigger system, however, still has the following defects. In the first place, a problem arises from unbalance of firing sensitivity between the two quadruple layer regions. In firing the quadruple layer regions by means of a light trigger signal, the regions where carriers are generated for effective firing are located in the vicinity of the reverse-biased intermediate junctions. If the firing sensitivity is to be improved, the light trigger signal is required to be radiated so as to reach the vicinity of the intermediate junctions. In the case where the two quadruple layer regions are to be fired by light radiated from one of the main surfaces of the semiconductor substrate, the distance between the light-receiving surface and the reverse-biased intermediated junctions where carriers effective for firing the quadruple layer regions are generated is different for the two quadruple layer regions by the thickness of the intermediate layer of the quintuple layers. In other words, light required for firing the quadruple layer region having an intermediate junction farther from the light receiving surface is attenuated as an exponential function by being absorbed into the intermediate layer, with the result that the light-firing sensitivity of that particular quadruple layer region is much lower than that of the quadruple layer region having the intermediate junction nearer to the light-receiving surface. To eliminate such unbalance between firing sensitivities, one measure will be taken in which light is irradiated on the side edge surface connecting the pair of opposite principal surfaces of the semiconductive substrate or another measure in which light is irradiated on both the opposite principal surfaces. Thus, a second problem arises. Namely, in accordance with the above one measure, for assuring the firing it is necessary to provide a light source of large irradiation area or two light sources, and in accordance with said another measure, not only two light sources are needed but also incorporation of two light sources encounters difficulties in view of the device structure. More particularly, in the former case, althrough electron-positive hole pairs are created by the light irradiation at an interface at which the two quadruple regions adjoin, these carriers will not flow into the outer layers but mainly will be transferred to the main electrodes from the intermediate layers without participating in the firing because the bidirectional thyristor is so constructed that the outer layer and an intermediate layer next thereto are short-circuited by the main electrode. Accordingly, in order that the bidirectional thyristor is fired by a light trigger signal, it is necessary to apply the light trigger signal to each of the quadruple layer regions at a portion remote from the interface between the two quadruple layer regions. Light is received at two or more points on the side surface, thus requiring a light source of large capacity having a large radiating area or two or more light sources. In the third place, prevention of the erroneous firing at commutating instant degrades the firing sensitivity or requires two light sources. In the bidirectional thyristor, an unfavorable phenomenon takes place that during so-called commutating in which one quadruple layer region is switched from conduction state to non-conduction state while the other quadruple layer region is switched from non-conductive state to conduction state, residual storage carriers of the one quadruple layer region interacts with the said other quadruple layer region with the result that the latter quadruple layer region is fired before a trigger signal is applied. To solve this problem, it has been known to form an isolation region devoid of the function of quadruple layer region between both the quadruple layer regions and to distribute a heavy metal atom substance on the isolation region to save the semiconductive substrate surface by reducing the width of the isolation region. With this construction, however, because of a short lifetime of carriers within the isolation region and for the reasons as mentioned above, there is need for the provision, for each of the quadruple layer regions, of light sources for trigger signals which are located remote from the isolation region. Present-day techniques have difficulties with distributing heavy metal substance only on the isolation region when the semiconductive substrate is relatively thick and usually, the heavy metal substance is distributed over the whole surface of semiconductive substrate. Accordingly, even if a light trigger signal is applied to any portion, short lifetime carriers fail to enable the firing unless the quantity of light is increased. On account of the above disadvantages, although rather superior to the electric trigger system, the light trigger system is not employed for triggering bidirectional thyristors of high blocking voltage and large current capacity.